Common-Vented Furnace and Water Heater in a Finished Basement
Final mechanical inspection of a 1962 split-level house. The contractor replaced an 80% AFUE, 100,000 Btu/h Category I furnace that has a draft hood, and left the existing 40,000 Btu/h atmospheric water heater. Both appliances connect to a single 5-inch Type B vent. The furnace connector is 24 inches of single-wall pipe; the water heater connector is 36 inches of single-wall pipe. Each enters a 5-inch double-wall tee. From the tee the vent runs 6 feet horizontally with two 90-degree elbows, then 18 feet vertically through an unheated attic to the roof. The single-wall connectors pass through a drywall ceiling in a framed opening with about 1/2 inch of clearance. Roof pitch is 4:12. The vent terminates 24 inches above the roof surface but only 6 feet horizontally from a second-floor operable window. A new 400 cfm bath exhaust fan was added in the adjacent bathroom. Rust staining is visible at the draft hood and at the water heater flue collar. The contractor says the arrangement has worked for 30 years and asks for a final sticker.
1. Identify the defects you would write up, and explain why each matters rather than merely citing a rule.
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The clearance is the clearest defect: single-wall connectors require 6 inches to combustibles and listed Type B vent requires 1 inch. A 1/2-inch framed opening through drywall satisfies neither, and the concealed ceiling penetration also means a connector is being run through a concealed space where only a listed vent should pass. The horizontal offset is a second issue: 6 feet of lateral with two 90-degree elbows consumes connector length and reduces the draft capacity of a 5-inch vent serving 140,000 Btu/h combined input. The installer must show that the vent and connectors were sized from the vent tables using the actual developed length and fitting allowances, not the old 5-inch pipe. The termination is suspect: 24 inches above a 4:12 roof is measured against the 10-foot horizontal rule, and the 6-foot distance to an operable window may not meet the required separation for a natural-draft termination. The rust and spillage stains are symptoms, not cosmetic: they indicate condensation, undersized venting, or depressurization from the new 400 cfm bath fan. A final cannot be approved on the basis of age.
2. Is common venting of these two appliances permissible here, and what would you require before deciding?
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Common venting is not automatically prohibited, but it is not a field decision to be made from pipe diameter alone. The appliances must be listed for common venting with each other, the manufacturer's instructions must permit it, and the common vent must be sized from the tables for the combined input, height, and connector geometry. A fan-assisted Category I furnace common-vented with a draft-hood water heater is a recognized configuration only when the listing and instructions allow it; many furnace manufacturers require a dedicated vent, and some draft-hood appliances cannot be common-vented with a power-vented or fan-assisted appliance. I would require the installation instructions and listing for both appliances, a vent-sizing calculation, and a combustion-safety test. That test should include a draft measurement at the draft hood and flue, a CO reading in the flue gas, and an orphaning test with the furnace operating and then off while the water heater runs. If the water heater spills when the furnace fan or the bath fan operates, the common vent is unacceptable regardless of pipe size.
3. State the correction sequence and the inspections needed before approval.
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I would not approve the final. The correction notice should require: replacing or properly shielding the connectors so clearances are maintained; removing connectors from concealed ceiling penetrations and using listed vent pipe where concealed; recalculating the vent and connector sizing, including the 6-foot offset and two elbows; and relocating or extending the termination to satisfy the roof-height and window-separation provisions. Because spillage is already evident, I would also require correction of the pressure problem, not just new pipe: verify that the bath fan does not depressurize the basement below the draft-hood spillage threshold, and provide make-up air if it does. The reinspection should include visual verification of clearances and supports, a draft test at each appliance, and a CO test. If the water heater is orphaned by a furnace change, it must be re-sized as a single-appliance vent. Only after those tests are documented would I consider the venting portion complete, and I would note that the electrical and gas connections still need their own approvals.